This invention relates to a projection lens used in a so-called projection television apparatus in which an enlargement of an image appearing on a cathode-ray tube is projected on a large screen.
The factors governing the quality of an image projected on a projection screen in such a projection television apparatus include the cathode-ray tube, projection lens and screen, and the present invention contemplates to improve the image quality by improving the structure of the projection lens. It is the recent tendency to change the material of the projection lens of the kind above described from the glass to a plastic material and provide aspherical lens surfaces so as to improve the brightness and resolution even with a small number of component lenses. Such a proposal is made in, for example, U.S. Pat. No. 4,300,817. The U.S. patent discloses a projection lens of three lens structure attaining an f-number of 1.0 thereby providing a very high brightness when compared with the past one. FIG. 1 shows schematically the structure of a projection television apparatus using such a projection lens.
The projection television apparatus shown in FIG. 1 includes a projection lens of three lens structure, and the reference numeral 1 designates a casing; 2, a cathode-ray tube; 3, a projection screen; 4, 5, 6, mirrors; 7, a having a concave surface lens; and 8, 9, having convex surfaces lenses.
Referring to FIG. 1, an image appearing on the cathode-ray tube 2 disposed in the casing 1 is magnified by the projection lens composed of the lens 7 and lenses 8, 9, and the magnified image is reflected by the mirrors 5, 6 to be projected and displayed on the projection screen 3 fixed to the casing 1.
Since the projection lens is composed of the lens 7 and lenses 8 and 9, a bright magnified image can be displayed on the projection screen 3. Further, since the optical path from the cathode-ray tube 2 to the projection screen 3 is bent by the mirrors 5 and 6, and, also, since the mirror 4 interposed between the lenses 8 and 9 having convex surfaces of the projection lens acts to bend the optical path, the projection television apparatus can be made compact and small in overall size.
However, although the lenses 7, 8 and 9 composing such a projection lens are configured to have predetermined spherical and aspherical surfaces to eliminate aberrations thereby improving the resolution, the resolution of the illustrated projection lens is not still fully sufficient or satisfactory. This is because a relatively large aberration occurs in the marginal zone of the image plane.
The contrast is another important item required for the image quality in such a projection television apparatus. The contrast is affected greatly by the stray light resulting from reflection by the lens surfaces, lens barrels, etc. The prior art projection lens disclosed in the cited U.S. patent has been defective in that the contrast is greatly degraded by the reflection of light from the concave lens surfaces.